Lecture 5: Nuclear Envelope Part 2

Function of the Nucleolus

1. Dense structure in the nucleus → responsible for rRNA biogenesis

2. rDNA found in nucleolus → transcribed into rRNA by Pol I → rRNA transcribed as 47S pre-rRNA → Spliced into 3 individual rRNA transcripts for ribosome biogenesis

3. RNAP II = transcribe DNA into mRNA | RNAPIII = transcribes tRNA | Pol IV and V = present in plants (transcribes miRNAs)

Functions of the Nucleoskeleton

1. Long filaments with coiled coil domains

2. Anchoring site for chromatin and inner nuclear membrane protein complexes → regulation of gene expression

3. Acts as primary anchroing site for the LINC complex

4. Provides structural scaffolding and nuclear morphology

Structure of the LINC Complex and its Function

Structure

1. Massive transmembrane structure made of outer nuclear membrane protein (KASH) and inner nuclear membrane protein (SUN)

2. SUN proteins (inner membrane; interacts with the nucleoskeleton) + KASH proteins (outer membrane; interacts with different cytoskeletal filaments) interact across the perinuclear space

Function

1. Nuclear morphology (connection to the cytoskeleton keeps the nucleus taut), chromatin organization, and gene expression

2. Nuclear movement

   1. Moving toward pathogen infection sites in root hair cells & movement in pollen cells during double fertilization

3. Communication with other organelles (i.e., relay of stress signals)

Name 2 Factors that Determine Chromatin Tethering to the Nuclear Surface

1. Nucleoskeleton: composed of lamins (in animals) or CRWN proteins (in plants), anchors peripheral chromatin to the inner nuclear surface

2. PNETs (Plant Nuclear Envelope Transmembrane Proteins): binds to the nucleoskeleton to anchor itself in the INM; directly tether heterochromatin to the nuclear surface

   1. Loss causes failed/inadequate development

Nuclear Envelope Dynamics During Open Mitosis

1. Initial degradation step: complete disintegration of the NPC before the rest of the membrane dissolves

2. Pre-segregation breakdown: occurs strictly before complete spindle formation and chromosome segregation

3. Post-segregation rebuilding: assembly of a new nuclear envelope and pores around segregated chromosomes in daughter cells